Bone tissue is a homogeneous material including osteoid and minerals. The osteoid is a viscous gel-like material including primarily type I collagen (approximately 90%), proteoglycans, and various sulfated and non-sulfated mucopolysaccharides. The mineral component consist primarily of a crystalline form of calcium phosphate, hydroxyapatite, with amounts of calcium carbonate, tricalcium phosphate, and smaller amounts of other forms of mineral salts. This bone tissue is laid down around cells called osteocytes and these cells are found in small interconnected channels (lacunnae) which are interconnected through a series of channels including the Haversian canal system. At the level of the microscope, it is possible to observe that bone tissue is organized into osteons of compact bone made of concentric, perivascular layers of highly coaligned mineralized collagen fiber bundles. The predominant orientation within a single layer varies with respect to the vascular axis and various combinations of orientation in successive lamellae and results in variable overall collagen orientation within each osteon. Differences in overall collagen orientation are directly reflected in differing mechanical behavior of single osteons. Transversely oriented collagen results in better resistance to compressive loading along the axis, whereas predominant longitudinal orientation results in better resistance to tensile stress. The predominant orientation of collagen within a cross-section of long bone is not random, but matches the expected distribution of mechanical stress across the section, and its rotational shift along the whole shaft. More transverse collagen is deposited at sites of compressive loading, and more longitudinal collagen is deposited at sites of tensile stress. These structural oriented bone tissues in a load-bearing bone are presumed to be laid down by the osteocytes present in the bone and bone remodeling mediates mechanical adaptation in compact bone.
A bone is typically comprised of bone tissue in the form of cortical and trabecular bone. Cortical bone is frequently referred to as compact bone and is the major load-bearing part of a bone. Trabecular bone is present in what is typically referred to as cancellous bone where it appears as densely interconnected structure of “spongy” bone. Spongy bone is typically bone that contains the hemotopoietic cellular elements which is called bone marrow. Trabecular bone can be described as forming a cross-bracing lattice between cortical bone in a bone. It is important to emphasize a need to differentiate between “a bone” and “bone” (as a tissue). A bone is comprised of bone tissue present as cortical and cancellous (spongy) bone.
The mineralized osteoid typical of bone tissue is hydrated along the organic molecular structure and is an essential element of the mineral structure. Hydrating molecules of water form complex molecular associations with these organic and non-organic elements of bone tissue and can be described as being tightly bound, loosely bound, and free. Free water and loosely bound water can frequently be removed from bone tissue with only minor changes in the overall mechanical characteristics of the bone tissue. Tightly bound water can be removed only under extreme conditions and results in significant changes in the physical and mechanical properties of bone tissue. In fresh bone, water serves a solvating function in bone tissue allowing proper orientation and molecular spacing of the collagen fibrils which maintain structural alignment of the mineral phase in association with the organic phase.
Bone tissue in the form of bone grafts for implantation into a patient, is typically preserved and provided in a dehydrated state. Dehydration of bone tissue through drying, whether by air drying or sublimation as in freeze-drying, results in alteration of the molecular structure of the bone tissue and as a result of the reorientation of the collagen fibrils and the crystalline mineral phase, stress accumulates in the bone tissue. This stress can be relieved by rehydration or by the occurrence of small or large dislocations of structure. Small dislocations are designated micro fractures and are not usually visible to the naked eye. Large dislocations are designated fractures and are usually visible to the naked eye.
In a long bone, for example a femur, tibia, fibula, or humerus, the shaft separates the proximal and distal ends of the long bone. The shaft serves to focus loads applied to the whole bone into a smaller diameter than found at the proximal and distal ends of the long bone and the shaft of a long bone is typically of a cylindrical shape and is comprised of compact (cortical) bone. Loads applied along the axis of the shaft require that the cortical bone maintains a constant circumference, i.e. the tendency to failure would distort the bone tissue perpendicular to the axis of load application. Thus, the orientation of the collagen fibers should be such that tensile stress is resisted along the axis of loading and compressive stress is resisted perpendicular to loading. Drying of shaft portions of long bones results in reorientation of collagen fibers and the mineral phase such that changes in the circumferential orientation create stress within the bone matrix which can be relieved only by rehydration or occurrence of a fracture which allows a reorientation approximating the original orientation. In dehydrated cortical ring grafts cut from the shafts of long bones, this stress release can present as a fracture along the long axis of the bone shaft leaving a circumference which approximates the circumference of the cortical ring graft prior to drying. By rehydrating bone grafts prior to implantation, the potential for fracture formation which can compromise the function of the bone product can be reduced, but not eliminated. Fractures as discussed above can occur in dehydrated bone prior to rehydration and result in a graft having compromised biomechanical properties, which in turn can result in graft failure when implanted in a patient.
Load-bearing soft tissue grafts such as ligaments, tendons, and fascia lata are frequently provided in a freeze-dried state. Such grafts must be rehydrated prior to clinical implantation. Such soft tissue grafts typically contain collagen, elastin, and assorted proteoglycans and mucopolysaccharides. The collagens and elastins are the load-bearing components of these soft tissue grafts and the assorted proteoglycans and polysaccharides serve to bind the fibrillar collagens into a matrix-like structure. The structural organization of fascia lata is similar to dura mater in being isotropic in load-bearing properties (Wolfinbarger, L., Zhang, Y., Adam, BLT., Hornsi, D., Gates, K., and Sutherland, V., 1994, “Biomechanical aspects on rehydrated freeze-dried human allograft dura mater tissues” J. Applied Biomaterials, 5:265-270) whereas tendons (for example the Achilles tendon) or ligaments (for example the Anterior cruciate ligament) are typically anisotropic in load-bearing properties. In these types of load-bearing soft tissue grafts, the tensile properties of the tissues depend on the flexibility of the collagenous structures to stretch under load and return to their original dimensions upon removal of the load.
A wide variety of bone and soft tissue products are used in veterinary, medical, orthopaedic, dental, and cosmetic surgery applications. These bone and soft tissue products can be used in load-bearing and non-load bearing applications and the bone and soft tissue products can be supplied under a variety of forms. Bone products are provided as fresh-frozen, freeze-dried, rehydrated freeze-dried, air-dried, organic solvent preserved, or provided preserved by other similar types of preservation methods. Each method of preservation of bone product possesses selected advantages and disadvantages and thus the method of preservation is generally modified to select for specific needs of a given bone graft. Soft tissue products are typically provided as fresh-frozen or freeze-dried and each method of preservation of soft tissue products possess selected advantages and disadvantages and thus the method of preservation is generally modified to select for specific needs of a given soft tissue product.
Bone and soft tissue products preserved and stored by methods involving freeze-drying (removal of water by sublimation) yield a bone or soft tissue product which is significantly more brittle than normal bone or soft tissue, and has a tendency to fracture into numerous small pieces, which ultimately can result in graft failure. Specifically, freeze-drying causes grafts to be brittle and typically causes shrinkage where the shrinkage is often not uniform, thereby causing graft failure; solvent preservation using for example, acetone or alcohol, can cause irreversible denaturation of proteins, and solubilization of solvent soluble components, including for example, lipids. These alterations in materials properties of the bone and soft tissue products necessitates a rehydration step in preparation of the bone and soft tissue product for implantation. However, rehydration does not solve the problem that grafts can fracture prior to rehydration, thereby making rehydration futile, and if there are micro fractures prior to rehydration they remain after rehydration. These grafts are more likely to fail regardless of whether or not they are rehydrated. Even after rehydration the materials properties do not approximate the materials properties of normal bone.
Bone and soft tissue products are generally separated into load-bearing and non-load-bearing products. Examples of non-load bearing bone products are ground demineralized bone which are used for inducing new bone formation in a particular implant site. Load-bearing bone products are rarely demineralized and are used at implant sites where the bone graft will be expected to withstand some level of physical loads. It is therefore important that load bearing bone products not fail during normal movements of the implant recipient and that the bone products not stimulate a pronounced physiological response. The majority of bone products are provided in either the fresh-frozen or freeze-dried format. The fresh-frozen format is undesirable because it includes donor derived bone marrow and is thus immunogenic and a source of disease transmission. The freeze-dried format is less of a problem than fresh-frozen grafts in the potential for disease transmission, however a freeze-dried bone graft is significantly more brittle than normal bone, more brittle than fresh-frozen bone, and must be rehydrated prior to clinical usage.
In that clinicians typically do not have time to adequately rehydrate bone graft products in the operating room, it is advantageous to provide a plasticized bone product which does not need rehydration, possesses adequate materials properties, and is not a potential source for disease transmission.